1. Field of the Invention
The present invention relates to a switching control technique for an optical signal exchanger, and in particular relates to a control apparatus and a control method of an optical signal exchanger that uses a reflecting tilt mirror made by micromachining (MEMS: Micro Electric Mechanical System) technology.
2. Description of the Related Art
Recently, with the increase in traffic on the Internet and the like, a demand for optical networks is increasing. Under such circumstances, attention is being paid to the introduction of optical signal exchangers that switch data of high speed and high volume just as in an optical signal state. As a conventional technique for realizing a high speed and high capacity optical signal exchanger, for example a system mechanically switching an optical fiber or a system constituted by combining waveguides, has been predominant. However, in this conventional technique, it is necessary to adopt a multistage constitution. Therefore, an optical loss inside the optical signal exchanger is very significant, and further, there is also a limit to deal with an increase in the number of channels. Consequently, it is difficult to realize an optical signal exchanger that deals with several 10 channels or more.
Under the abovementioned circumstances, an optical switch using a tilt mirror (hereunder referred to as an MEMS mirror) made by applying micromachining (MEMS) technology is predominant compared to other switches, from the point of miniaturization, wavelength independence and polarization independence, and is thus gaining attention. In particular, for example as shown in FIG. 22, an optical signal exchanger of three-dimensional type constituted by combining two collimator arrays 1A and 1B having a plurality of collimators arranged in two dimensions, respectively, and two MEMS mirror arrays 2A and 2B having a plurality of MEMS mirrors arranged in two dimensions, respectively, is expected from the point that a reduction in optical loss, a large capacity and multichannel can be realized.
Regarding the abovementioned three-dimensional optical signal exchanger, the present applicant has proposed a control technique for automatically correcting angular displacement of respective MEMS mirrors to reduce an optical loss (Japanese Unexamined Patent Publication No. 2002-236264). A control apparatus for an optical signal exchanger applied with this control technique, for example as shown in FIG. 23, automatically corrects the angular displacement of reflecting surfaces of respective MEMS mirrors by; detecting in an optical power detection section 12, power of light branched by an optical coupler array 11 provided on a latter stage of an output optical fiber array 10B connected to a collimator 1B on an output side, judges in a comparison control section 13 based on the detection results, coupling states of optical signals with respect to output optical fibers, and controlling respective MEMS mirror drive sections 14A and 14B so that the loss inside the optical signal exchanger become minimum.
However, in this control technique for minimizing the loss inside the optical signal exchanger, when performing the channel switching so that for example an optical path transmitting an optical signal from an input point A to an output point B, is changed to an optical path transmitting an optical signal from an input point Axe2x80x2 to an output point B, then as shown in a conceptual diagram of FIG. 24, if an optical input level to the input point A is different from that to the input point Axe2x80x2, a level of the optical signal to be output to an identical output point B, is changed before and after the switching. There is a possibility that such a change in the optical output level accompanying the channel switching influences on a system connected to a latter part of the optical signal exchanger. More specifically, this is likely to cause saturation of an optical amplifier or an increase in a bit error rate, or the like.
The present invention has been accomplished in view of the abovementioned problems, with the object of providing a control apparatus and a control method of an optical signal exchanger, capable of controlling an optical output level to be constant even when performing the switching of channels with different optical input levels.
In order to achieve the abovementioned object, according to the present invention, there is provided a control apparatus of an optical signal exchanger which includes a first mirror array and a second mirror array, each having a plurality of tilt mirrors arranged on a plane, each tilt mirror having a reflecting surface an angle of which is controllable, and which sequentially reflects an input optical signal by the first and second mirror arrays to output from a specific position, for controlling the angle of the reflecting surface of each of the tilt mirrors of the first and second mirror arrays, comprising: an optical power detection unit that detects power of the optical signal output from the specific position; and an angle control unit that controls the angle of at least one of the reflecting surfaces of the tilt mirrors of the first and second mirror arrays, which have reflected the optical signal, so that the optical power detected by the optical power detection unit becomes constant at a target value set corresponding to the specific position.
With such a constitution, the power of the optical signal sequentially reflected by the first and second mirror arrays to be output from the specific position is detected by the optical power detection unit, and the angle of the reflecting surface of one or both of the tilt mirrors positioned on a propagation path of the optical signal is controlled by the angle control unit so that the output light power becomes constant at a previously set target value. As a result, even when the switching is performed on channels with different optical input levels, the optical output level can be controlled to be constant regardless of a change in the optical input level.
Moreover, as a specific constitution for the abovementioned control apparatus, the angle control unit may comprise: a first mirror drive section that changes stepwise the angle of the reflecting surface of each tilt mirror of the first mirror array in a constant control direction; a second mirror drive section that changes stepwise the angle of the reflecting surface of each tilt mirror of the second mirror array in a constant control direction; and a comparison control section that calculates absolute values of differences between the target value and respective values of the output light power that are detected by the optical power detection unit immediately before and after the angle of the reflecting surface is changed by at least one of the first mirror drive section and the second mirror drive section to compare the absolute values with each other, and determines respective control directions in the first mirror drive section and the second mirror drive section based on the comparison result, to feedback control the angle of the reflecting surface so that the absolute values of the differences become minimum. With such a constitution, the angles of the reflecting surfaces of the respective tilt mirrors are feedback controlled based on the absolute value of the difference between the value of the output light power detected by the optical power detection unit and the target value.
Further, as a preferred aspect of the abovementioned control apparatus, the comparison control section may, for respective axes of the reflecting surfaces of the respective tilt mirrors of the first and second mirror arrays, in an initial state before feedback controlling the angles of the reflecting surfaces, investigate to determine the control directions where the value of the output light power detected by the optical power detection unit approximates to the target value, and sequentially switch the feedback control for each axis in accordance with each of the determined control directions. According to such a constitution, after the directions in which the respective axes are to be controlled are preliminarily investigated and determined, the angles of the reflecting surfaces are feedback controlled in accordance with the control directions.
As another preferred aspect of the abovementioned control apparatus, the comparison control section may, for respective axes of the reflecting surfaces of the respective tilt mirrors of the first and second mirror arrays, in an initial state before feedback controlling the angles of the reflecting surfaces, initially set the angles of the reflecting surfaces so that the optical loss with respect to the optical signal output from the specific position becomes minimum, and then feedback control in accordance with an arbitrary control direction with respect to at least one axis of the plurality of axes. According to such a constitution, it becomes unnecessary to perform the above described investigation of the control directions for the respective axes, and hence a control time can be shortened.
The present invention further provides a control method of an optical signal exchanger which includes a first mirror array and a second mirror array, each having a plurality of tilt mirrors arranged on a plane, each tilt mirror having a reflecting surface an angle of which is controllable, and which sequentially reflects an input optical signal by the first and second mirror arrays to output from a specific position, for controlling the angle of the reflecting surface of each of the tilt mirrors of the first and second mirror arrays, comprising the processes of: detecting power of the optical signal output from the specific position; and controlling the angle of at least one of the reflecting surfaces of the tilt mirrors of the first and second mirror arrays, which have reflected the optical signal, so that the optical power detected becomes constant at a target value set corresponding to the specific position.
Other objects, characteristics and advantages of the present invention will become apparent from the following description of embodiments, in conjunction with the appended drawings.